In this patent application, the term ‘soil layer’ is used in the sense of a quantity of soil material located below the ground surface which can be distinguished from adjacent soil material by its specific composition or texture, such as a clay layer or a sand layer, for example, which consist mainly of clay or sand, respectively. The term soil layer therefore also refers to a specific soil type. A soil layer or soil of course also comprises the soil water which is present therein. Treatment of soil therefore also comprises treatment of the groundwater present in the soil.
U.S. Pat. No. 4,659,259 describes a method for stabilizing an unstable clay layer, using a soil auger having a helical drilling blade in which outlet openings are provided. A treatment product which chemically reacts with clay is injected into the clay layer via the outlet openings and is mixed with the clay through the rotation of the helical blade. As a result of the chemical reaction, the clay layer is locally stabilized. The treatment product is ideally injected over the entire depth of the unstable clay layer.
A method having the features of the first paragraph of this description is also used in the remediation of polluted soil. A liquid or gaseous treatment product containing chemical products or bacteria or activated carbon, etc. which can render the pollutants in the soil or groundwater harmless is injected into the soil. Taking into account the results of prior soil investigations and potentially various parameters, including the identified concentrations of polluting parameter(s), the possible presence of degradation or daughter product(s), the geochemical state of the soil layers, the distance from the source of the pollution, the time that has passed since the moment at which pollution took place, the nature and the depth of the different soil layers in the subsoil and the direction of the groundwater flow, it is determined where and in which soil layer or soil layers the treatment product must be injected in order to render as many of the pollutants as possible harmless. In places relatively close to the source of pollution, it may thus be most efficient to inject a treatment product into the soil layer which has the smallest hydraulic conductivity at that location in comparison with the other soil layers, such as for example a clay layer. In places further away from the source of the pollution, it may then be more efficient to inject the treatment products into the soil layer which has the largest hydraulic conductivity in comparison with the other soil layers, such as for example a sand layer. When polluting products move in a certain direction underground with the groundwater flow, a treatment product can be injected into well-defined soil layers which lie on the trajectory of the groundwater flow before the pollutants have moved there, in order to form a barrier that may prevent the further movement thereof.
A method for treating polluted soil is described in US 2014/0231322 A1. This method has the properties indicated in the first paragraph of this description and provides for the use of a rod-shaped injection element with a conical head and smooth outer walls. The injection element comprises a measuring instrument in cooperation with a sensor for collecting data on the pollutants present in the soil or the soil water at various depths, and an injection part for injecting a treatment product in order to render the detected pollutants harmless. Based on the data on the pollution collected via the sensor, a control unit determines the dose of the treatment product at the various depths. The injection pressure is also controlled as a function of the data on the pollution.
As already stated above, it is of great importance in a number of soil treatments that the treatment product is predominantly injected into one or more well-defined soil layers and the injection in some treatments must also be as homogeneous as possible over the entire depth trajectory or as far as possible over the entire depth of said well-defined soil layer. In this case, it is also very important that the correct quantity of treatment product (the quantity required to achieve an effective treatment) can reach the respective soil layers with a high level of certainty.
The method known from US 2014/0231322 A1 moreover has yet another drawback. As the rod-shaped injection element is pushed or hammered into the soil, the soil around it is compacted and smeared. These phenomena of ‘compaction’ and ‘smearing’ considerably reduce the permeability of the soil at the location of the outlet openings of the injection element and make injection more difficult, as a result of which higher injection pressures need to be used to achieve a workable injection flow rate. A possible consequence of these higher injection pressures is that the injected treatment product causes undesired hydraulic fracturing of the soil, as a result of which the injected treatment product is distributed into the soil via a number of preferred paths, which is of course detrimental to the efficiency of the treatment, and even ends up at the surface via such preferred paths. Pushing or hammering the rod-shaped injection element into the ground is moreover also associated with lateral vibrations, thus forming a channel around the injection element and the extending rods above it. The injected treatment product can relatively easily be pushed upwards along the injection element via this channel (referred to below using the technical term ‘blow-out’).
As the injection element is pushed or hammered into the soil and as it is not possible to simultaneously push and inject, the treatment product cannot be injected into the soil at every desired depth (i.e. over the entire desired depth trajectory), and this is done, for example, only at certain discrete depths with a vertical intermediate distance of approximately 30 cm.
As a result of one or more of the drawbacks mentioned, the injected product thus very often does not reach the different soil layers intended, or only in a much smaller quantity than required. As a result, the treatment is much less efficient than expected or the processes intended with the injected product do not take place at all.
Often, investigations are carried out in advance into the distribution of the pollution in the soil by means of soil augering with associated soil sampling and analyses, monitoring wells and associated groundwater sampling and analyses and MIP/Enissa MIP probing. This allows the spread of the pollution in the soil to be mapped. These techniques can also be used to acquire an idea of the geology but only limited information is provided as to the hydraulic permeability and the injectability of the different soil layers. Another instrument used for this is known under the commercial name ‘hydraulic profiling tool’ or ‘HPT’. A rod-shaped element with a conical point and a smooth outer side is pushed or hammered into the soil. Via a number of outlet openings in the rod-shaped element, water is injected into the soil at a low and known flow rate. Based on measurements of the injection pressure, the hydraulic conductivity can be assessed at different depths. Such preliminary soil investigation is cumbersome and time-consuming and, due to the limited volume of water injected, it can often be difficult to extrapolate the results obtained to injectability of the soil layers concerned. Moreover, the measurement values often give a distorted picture of the actual soil properties. During the insertion of the rod-shaped element into the soil, the soil is compacted by the conical point, and the soil pores are smeared closed by the smooth outer sides of the rod-shaped element. This ‘compaction’ and ‘smearing’ reduce the permeability of the soil at the location where the water is injected. As a result, the hydraulic conductivity is often underestimated, especially when injecting into heavier soil types, such as clay, loam, sandy loam and loamy sand.